The present invention relates to a packet data transmission system and further relates to primary and secondary stations for use in such a system and to a method of operating such a system. The present invention has particular, but not exclusive, application to UMTS (Universal Mobile Telecommunication System) otherwise referred to as 3GPP.
For packet radio in UMTS it has been proposed for High Speed Downlink Packet Access (HSDPA) that MIMO (Multiple Input Multiple Output antenna systems) be used. One example of a MIMO system is described in U.S. Pat. No. 6,067,290 in which the presence of multiple antennas enables spatial multiplexing whereby a data stream for transmission is split into a plurality of sub-streams, each of which is sent via many different paths. Another example, known as the BLAST system, is described in the paper “V-BLAST: an architecture for realising very high data rates over the rich-scattering wireless channel” by P W Wolniansky et al in the published papers of the 1998 URSI International Symposium on Signals, Systems and Electronics, Pisa, Italy, 29 Sep. to 2 Oct. 1998.
The performance gains which may be achieved from a MIMO system may be used to increase the total data rate at a given error rate, or to reduce the error rate for a given data rate, or some combination of the two. A MIMO system can also be controlled to reduce the total transmitted energy or power for a given data rate and error rate.
One area in which MIMO techniques may be applied is a High-Speed Downlink Packet Access (HSDPA) scheme, which is currently being developed for UMTS and which may facilitate transfer of packet data to a secondary station, for example a mobile station, at over 4 Mbps. In one proposed embodiment of HSDPA separate data streams using the same channelisation code are sent from respective antennas at a primary station, for example a base station, and the data streams can in principle be received and decoded by a secondary station having at least as many antennas as there are data streams. An ARQ (Automatic Repeat reQuest) scheme is needed to ensure correct delivery of each data packet, since accurate data transmission is viewed as more important than the reduced system throughput under poor channel conditions (due to multiple re-transmissions).
A problem with the use of a MIMO system for packet data transmission is the impact of differing radio link qualities on the communication system. The scheme can perform well with low interference levels but not with high interference levels such as will occur near a cell border where interference from primary stations in adjacent cells will greatly reduce throughput. It is an object of the present invention to mitigate the effect of such interference in packet data transmission.